1. Field of the Invention
The present invention relates to a document sheet recognition device for recognizing position and size of a document sheet in a copying machine or the like.
2. Description of the Prior Art
In office equipment which handles document sheets, such as an electrophotographic machine or a facsimile machine, a variable magnification optical system is often mounted to provide variable magnification as one of the machine's functions. When the variable magnification optical system is used in the electrophotographic machine, a magnification may be selected in accordance with a size of an original document so that an image of the original document is exactly copied onto a copy sheet of a given size. In order to precisely carry out this operation, an apparatus to recognize the size of the original document and, in some cases, the position thereof, is required. In an apparatus having means to recognize the size or position of the original sheet, it is possible to first recognize the size and position of the document sheet and then select a copy sheet which fits the size of the original document sheet.
A prior art apparatus of this type is constructed as shown in FIG. 1. A unit A which comprises a document sheet 1, a press plate 3 and a document sheet glass 2 is moved in an x-direction relative to other members. The movement of the unit A may be effected by mechanical movement of the unit itself or by mechanical movement of a reflection mirror system which directs light to a focusing lens 4 (stationary document sheet system). Other members include document sheet illumination optical system 10, focusing optical system 4, photo-acceptor 5 such as photo-conductive drum, developing unit 7 for electrophotographic processing, optical system 8 for recognizing position and size of the document sheet, linear photo-diode array 9 and driver 11. A surface of the press plate 3 is a mirror surface. Light irradiated from the light source 10 to the press plate 3 is prevented from impinging on the focusing optical system 4 or the optical system 8 for recognizing the position and size of the document sheet. Only diffused light flux from the surface of the document sheet is directed to those optical systems.
The document sheet position and size recognition means can recognize the area of the document sheet shown in FIG. 2 as the unit A moves. A hatched area shows an area of the unit A in which the document sheet is not present.
The photo-diode array 9 can detect a light intensity distribution in a y-direction of the unit A. FIG. 3 shows the distribution. When an output P of the photo-diode array exceeds a threshold S.sub.K, it is recognized that the document sheet is present in an area of the unit A which corresponds to the address at which the threshold for the photo-diode array 9 was exceeded. In FIG. 3, the document sheet exists in the area between the addresses 6 and (n-1), inclusive. This step is sequentially repeated as the unit A is moved in the x-direction so that the area of the document sheet in the unit A is detected.
The size and position of the document sheet can be detected in this manner. However, the prior art method requires formation of focusing light flux from the document sheet to the focusing optical system 4 as well as focusing light flux from the document sheet to the photo-diode array 9. Since signals at all addresses of the photo-diode array 9 should be evenly processed, the required technique is complex. High speed signal processing is required in order to detect the presence of the document sheet for each address in the y-direction while effecting the x-direction movement. Since the spatial resolution is determined by the total number of addresses of the photo-diode array, a photo-diode array having a number of addresses is required to achieve a high resolution. When such means is used, the signal processing speed should be further increased. When the light source 10 is energized by an A.C. voltage such as a fluorescent lamp, illumination light intensity varies in time and detection precision is lowered.